Comparative foliar water uptake and leaf hydrophobicity among eight species of California ferns

Background/Question/Methods

In 2014, Southern California experienced the worst drought in the last 1,200 years.  Recent tree ring analysis and climate models suggest that this event may be the front end of a “mega-drought” in the southwestern United Sates. Ferns might be particularly vulnerable. In this study, the mechanisms of drought tolerance for eight species of ferns in the Santa Monica Mountains were assessed with a focus on foliar water uptake and hydrophobicity of leaf surfaces. We hypothesize that foliar water uptake allows some fern species to persist by utilizing morning fog as a moisture source.  This hypothesis was tested using a 4-digit analytical balance for gravimetric analysis and a leaf area meter to normalize water uptake rates.  A Scholander-Hammel pressure chamber was used to prescribe equal hydration levels among species and estimate minimum seasonal water potentials Ψ (MPa). Leaf hydrophobicity was estimated by measuring the contact angle between water droplets and leaf surfaces.  We also surveyed the resurrection process of a desiccation tolerant species by artificial irrigation, in situ.  Chloroplast function was monitored non-invasively using a pulse-modulated fluorometer and native embolism estimated by measuring hydraulic flow before and after embolism removal.

Results/Conclusions

Foliar water uptake (g/m² min) was correlated with minimum seasonal water potential Ψ (MPa) (r² = 0.71).  This suggested that foliar water uptake might assist dehydration tolerant species in critical water balance during severe drought. Also, leaf hydrophobicity correlated with foliar water uptake (r² = 0.52), suggesting that ferns inhabiting drier microsites are more capable of absorbing atmospheric water through their foliage than ferns in or adjacent running water. The monitoring of rapid recovery in desiccation tolerant species, in situ, revealed that height of uncurled pinnae was correlated with days since initial irrigation (r² = 0.97) as well as diameter of uncurled pinnae (r² = 0.98).  We examined embolism reversal as a potential mechanism of desiccation recovery. Preliminary data suggested that native ferns are 100% embolized whereas irrigated ferns are only 40% embolized.  This suggests embolism reversal may serve as a drought survival mechanism in desiccation tolerant species.  Taken as a whole, this study deepens our understanding of how primitive ferns may cope with an increasing probability of unprecedented mega-drought in California as well as the Southwestern United States.